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Anode erosion of a moisture probe

  1. Mar 29, 2013 #1
    I'm working on a project in which I'm using a basic moisture sensor that I've made by sliding an insulated wire inside of a copper tube in such a way that the insulated wire with it's insulation extends past the tube about 1/8". That is then poked into the ground. On the other end of the copper tube I connect a wire with a 2k resistor to ground. I send 5 volts through the wire and take a voltage reading at a point before the 2k resistor - a basic voltage divider. All that's fine and is not a problem. When the ground is moist I get a reading and when it's not I get zero volts because what voltage does make it through goes to ground.

    The problem I'm having is the decay of the anode (the copper wire tip). In this case, the copper wire is acting as an anode, with the soil moisture (because of the salt content) acting as an electrolyte. I tried various metals to replace the copper wire and they all eventually erode. So then I replaced it with piece of .5mm mechanical pencil lead (graphite) which I crimped into a tiny copper tube and sealed with petroleum jelly and shrink wrap to protect the copper tube from moisture so that all anode activity is upon the graphite that extends past the shrink wrap and wa-la, no corrosion and all works great. The only problem is the graphite tip is too fragile so it can be easily broken off.

    My question for you all, since I know little about the ion exchange or whatever is going on to deteriorate all but the graphite tip, is what else can I use that would conduct electricity in the same way to form my voltage divider circuit without being fragile like the graphite yet also not corrode.

    You can kinda see how I'm using this in one of my YouTube videos at http://www.youtube.com/watch?v=QembRhdR4z0&list=PLzZP3kZn2OzMWUHs1xM0R-waD2ALm87uj&index=2 If you go to 2:14, you'll see a typical moisture meter probe which I started with before using the copper wire inside the tube, both of which preceded the use of the graphite. If you go to the picture at 2:34 and look to the lower right hand side of the controller box, you'll see the probe inserted through the wood frame into the soil.

    Please help, or direct me to a place in this forum where I might receive a solution.

  2. jcsd
  3. Mar 29, 2013 #2
    Too much current?

    How about increasing your 2k resistor to 200k? That would slow the anode corrosion by a factor of 100.
  4. Mar 29, 2013 #3
    That's actually a pretty good idea that somehow eluded me - thanks.
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